1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor optical functional device used in optical communications systems and optical information systems.
2. Description of Related Art
Up to now, semiconductor optical functional devices corresponding to light in the long wavelength band used in optical communications have primarily had a BH (buried hetero) structure, as discussed in Publication 1, for example (Publication 1: xe2x80x9cNative-Oxidized InAlAs Blocking Layer Buried Heterostructure InGaAsP-InP MQW Laser for High-Temperature Operation,xe2x80x9d IEEE Photonics Technology Letters, Vol. 11, No. 1, January 1999). However, the manufacture of devices with a BH structure requires a crystal growth step to be repeated a number of times, and furthermore the manufacturing process is somewhat complicated. In view of this, considerable effort in recent years has gone into the development of semiconductor functional devices with a ridge-type structure, which require only a single crystal growth step and can be manufactured more easily than a device with a BH structure, as discussed, for example, in Publication 2 (Publication 2: xe2x80x9cInP-Based Reversed-Mesa Ridge-Waveguide Structure for High-Performance Long-Wavelength Laser Diodes,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, Vol. 3, No. 2, April 1997).
With these semiconductor functional devices having a ridge-type structure, a plurality of reversed-mesa structures are successively formed adjacent to each other. A polyimide resin is provided not only in between these ridge-type reversed-mesa structures, but also in contact with the side walls of the reversed-mesa structures on both sides of the reversed-mesa structures in order to reduce parasitic capacitance and make the device flatter. Annealing is performed in the step in which this polyimide resin is provided in order to cure the polyimide resin that coats the upper surface of a semiconductor wafer. Heat causes the polyimide resin to expand or contract during this annealing, and this subjects the side walls of the reversed-mesa structure to stress. If an active layer is provided in the vicinity of the side walls of the reversed-mesa structure, this stress may have an adverse effect on the reliability of the optical functional device.
There are also semiconductor optical functional devices with a ridge-type structure that have what is known as an air bridge, in which the upper surface of one mesa portion and the upper surface of an adjacent mesa portion are bridged by an electrode pad, without both sides of the mesa portions being covered with a polyimide resin.
In devices having an air bridge, though, there is the danger of damage to the device occurring (in which the electrode pads may break, or the mesa portions snap) in the step of polishing the back of the wafer, for instance, in the course of wire bonding or junction down mounting.
Therefore, it is an object of the invention to provide a method for manufacturing a device which does not subject the side walls of the mesa portion to stress.
To achieve the stated object, the method of the present invention for manufacturing a semiconductor optical functional device comprises:
(1) forming a laminated semiconductor layer over a substrate;
(2) forming an island-form preliminary pattern whose side wall surface is substantially perpendicular to the upper surface of the substrate by patterning all or part of the laminated semiconductor layer;
(3) forming an insulating material component on the top side of the substrate so that the upper surface of the preliminary pattern and part of the side walls of the preliminary pattern are exposed; and
(4) etching the side walls of the preliminary pattern and thereby changing this preliminary pattern into a reversed-mesa structure component that contributes to optical function and forming a space between the reversed-mesa structure component and the insulating material component.
In the above-mentioned (1), a semiconductor layer that will subsequently become the reversed-mesa structure component that contributes to optical function is formed over a substrate, and then in (2) this semiconductor layer is patterned to obtain a preliminary pattern. The side wall surface of this preliminary pattern is substantially perpendicular to the upper surface of the substrate, and this is called a preliminary pattern because it is not in the form of a reversed mesa. The upper surface of this preliminary pattern has substantially the same size and shape as the upper surface of the reversed-mesa structure component. Next, in (3), the insulating material component is formed on the sides of the preliminary pattern. This insulating material component serves to flatten the device, for example. When a thermoplastic resin such as a polyimide is used as the material that makes up this insulating material component, for example, the sides of the preliminary pattern are coated with this material and then heated and cured during this (3).
Next, in (4), the side walls of the preliminary pattern are etched so as to change the preliminary pattern into a reversed-mesa structure component. In order to etch the side walls of the preliminary pattern, the insulating material component is formed in (3) so that part of the side walls of the preliminary pattern will be exposed from the insulating material component. A reversed-mesa structure component is then obtained by etching the entire side wall the portion where the side wall of the preliminary pattern is partially exposed. This etching forms a space, which widens from the upper surface toward the substrate, between the reversed-mesa structure component and the insulating material component. As a result, the area around the neck portion of the reversed-mesa structure component is covered by air, so there is no danger of strain occurring.